Low drive voltage light emitting element

ABSTRACT

An object of the present invention is to provide a material which does not substantially have a hole injection barrier from an electrode. A composite material containing an organic compound and an inorganic compound, in which measured current-voltage characteristics of a thin-film layer formed from the composite material which is sandwiched between a pair of electrodes each having a work function of 3.5 eV to 5.5 eV follow Formula (1) below, is manufactured. J = { A ⁢ ⁢ exp ⁡ ( - φ a 2 ⁢ kT ) } ⁢ V + BV n ( 1 )

TECHNICAL FIELD

The present invention relates to a composite material in which anorganic compound is compounded with an inorganic compound and which canform a favorable contact with various kinds of electrodes. The inventionalso relates to a current-excitation light emitting element in which thecomposite material is provided in contact with an electrode.

BACKGROUND ART

In recent years, a light emitting element using a light emitting organiccompound has been actively researched and developed. A basic structureof this light emitting element is that a layer containing a lightemitting organic compound (light emitting layer) is sandwiched between apair of electrodes. By applying a voltage to this element, electrons andholes are separately transported from the pair of electrodes to thelight emitting layer, and current flows. Then, recombination of thesecarriers (the electrons and holes) makes the light emitting organiccompound to form an excited state and to emit light when the excitedstate returns to a ground state. Owing to such a mechanism, such a lightemitting element is referred to as a current-excitation light emittingelement.

Note that an excited state of an organic compound includes a singletexcited state and a triplet excited state. Light emission from thesinglet excited state is referred to as fluorescence, and light emissionfrom the triplet excited state is referred to as phosphorescence.

A great advantage of such a light emitting element is that the lightemitting element can be manufactured to be thin and lightweight, sincethe light emitting element is generally formed of an approximatelysubmicron thin film. In addition, extremely high response speed isanother advantage, since time between carrier injection and lightemission is approximately microseconds or less. These characteristicsare considered suitable for a flat panel display element.

Such a light emitting element is formed in a film shape. Thus, surfaceemission can be easily obtained by forming a large-area element. Thischaracteristic is hard to be obtained by a point light source typifiedby an incandescent lamp or an LED or a line light source typified by afluorescent lamp. Therefore, the above described light emitting elementhas high utility value also as a surface light source applicable tolighting or the like.

Thus, the current-excitation light emitting element using the lightemitting organic compound is expected to be applied to a light emittingdevice, lighting, or the like. However, there are still many problems.As one example of the problems, reduction in power consumption is given.It is an important issue to reduce a drive voltage of the light emittingelement in order to reduce power consumption. Since emission intensityof the current-excitation light emitting element depends on the amountof current flowing therethrough, it is necessary to conduct a largeamount of current at low voltage in order to reduce a drive voltage.

It has been attempted so far to provide a buffer layer in contact withan electrode as a technique for reducing a drive voltage. Specifically,it is known that a drive voltage can be reduced by providing a bufferlayer using an aromatic amine compound at an interface with an anode(for example, Reference 1: Y. Shirota et al., Applied Physics Letters,Vol. 65, 807-809 (1994)). The aromatic amine compound used in Reference1 has a high location of HOMO level and an approximate value to a workfunction of an electrode material for forming the anode. Therefore, ahole injection barrier can be lowered. Accordingly, a large amount ofcurrent can flow at relatively low voltage.

Another method is also reported, in which a layer, conductivity of whichis increased by adding electron-accepting molecules to a holetransporting high molecular weight material, is used at an interfacewith an anode (for example, Reference 2: A. Yamamori et al., AppliedPhysics Letters, Vol. 72, 2147-2149 (1998)). A drive voltage can also bereduced by using such a structure.

However, there is a problem in that such an organic compound which canlower a hole injection barrier as described in Reference 1 is limited,and heat resistance of the material is generally not high. The sameapplies to such an electron accepting molecule as described in Reference2.

Conventionally, even if an organic compound which can lower a holeinjection barrier is used, the hole injection barrier cannot be made todisappear substantially, and current-voltage characteristics of thelight emitting element are controlled by injection (in other words,current-voltage characteristics in which a Schottky injection mechanismis dominant). Therefore, there is limitation on further reduction in adrive voltage.

Further, when a material which does not have a high work function isused for an anode, a hole injection barrier thereof is more increased.Therefore, there is another limitation in that a material having a highwork function needs to be used as an electrode material for forming theanode in order to prevent an increase in drive voltage of the lightemitting element. In other words, this leads to a problem in thatgeneral-purpose metal such as aluminum, which does not have a high workfunction, cannot be used for the anode.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a material whichdoes not substantially have a hole injection barrier from an electrode.It is another object of the invention to provide a material which doesnot substantially have a hole injection barrier with various electrodes.It is still another object to provide a material having thesecharacteristics and high heat resistance.

In addition, it is also an object to provide a light emitting elementand a light emitting device with a low drive voltage by using thematerial. It is yet another object to provide an inexpensive lightemitting element and an inexpensive light emitting device by using acombination of the material and general-purpose metal.

As a result of keen examination, the present inventor has found that theobjects can be achieved by providing a composite material, in which anorganic compound is compounded with an inorganic compound, in contactwith an electrode of a light emitting element. One feature of thiscomposite material is that measured current-voltage characteristics atthe time of using an electrode having a work function of 3.5 eV to 5.5eV are expressed by addition of current showing behavior like animpurity semiconductor (i.e. ohmic current which easily flows whentemperature increases) to trap-charge limited current.

It is found that the composite material showing such current-voltagecharacteristics does not substantially have a hole injection barrierwith an electrode having a work function of 3.5 eV to 5.5 eV, which isdifferent from a conventional material formed from only an organiccompound. In addition, the composite material also has high heatresistance since an inorganic compound is compounded.

Thus, one structure of the invention is a composite material containingan organic compound and an inorganic compound, in which measuredcurrent-voltage characteristics of a thin-film layer formed from thecomposite material which is sandwiched between electrodes each having awork function of 3.5 eV to 5.5 eV follow the following Formula (1).

$\begin{matrix}{J = {{\left\{ {A\;{\exp\left( \frac{- \phi_{a}}{2{kT}} \right)}} \right\} V} + {BV}^{n}}} & (1)\end{matrix}$

(J denotes a current density; V, a voltage; φ_(a), activation energy forcarrier generation in the composite material; k, Boltzmann constant; T,a temperature; A and B are parameters determined by a distance betweenthe pair of electrodes d, the amount of elementary charge q, a mobilityμ determined by the kind of the composite material, a dielectricconstant ∈, the number of traps per unit volume Nt, the number of LUMOlevels per volume of the organic compound in the composite materialN_(LUMO); and n, an integer of 2 to 10.)

Note that, at this time, φ_(a) is preferably 0.01 eV to 0.5 eV. Inaddition, a thickness of the thin-film layer at the time of measuringthe current-voltage characteristics is preferably 10 nm to 500 nm.

The inorganic compound preferably shows an electron-accepting propertyto the organic compound. In particular, many transition metal oxidesshow highly electron-accepting properties, and among them, titaniumoxide, zirconium oxide, hafnium oxide, vanadium oxide, niobium oxide,tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, or rhenium oxide is suitable.

On the other hand, the organic compound preferably has ahole-transporting property. In particular, many aromatic amine compoundsshow highly hole-transporting properties, which are suitable. Inaddition, the aromatic amine compound is preferable in that the aromaticamine compound easily donates electrons to the inorganic compoundshowing an electron-accepting property.

A light emitting element with a low drive voltage can be obtained byproviding the composite material of the invention in contact with anelectrode of the light emitting element. In addition, as describedabove, the composite material of the invention does not substantiallyhave a hole injection barrier with an electrode of 3.5 eV to 5.5 eV.Thus, general-purpose metal having a work function in this range can beused as the electrode of the light emitting element. Accordingly, aninexpensive light emitting element can be provided.

Therefore, another structure of the invention is a light emittingelement including a first layer and a second layer containing a lightemitting material between a first electrode and a second electrode, inwhich the first layer is provided in contact with the first electrode,and the first layer is formed from the above-described compositematerial of the invention.

In addition, another feature of the light emitting element of theinvention is that an electrode and a layer in contact with the electrode(i.e. a layer using the composite material of the invention) form anohmic contact with each other. In other words, another structure of theinvention is a light emitting element including a first layer containingan organic compound and an inorganic compound and a second layercontaining a light emitting material between a first electrode and asecond electrode, in which the first layer is provided in contact withthe first electrode, and the first electrode forms an ohmic contact withthe first layer.

At this time, the inorganic compound preferably shows anelectron-accepting property to the organic compound. In particular, manytransition metal oxides show highly electron-accepting properties, andamong them, titanium oxide, zirconium oxide, hafnium oxide, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, or rhenium oxide is suitable.

On the other hand, the organic compound preferably has ahole-transporting property. In particular, many aromatic amine compoundsshow highly hole-transporting properties, which are suitable. Inaddition, the aromatic amine compound is preferable in that the aromaticamine compound easily donates electrons to the inorganic compoundshowing an electron-accepting property.

Note that the first electrode is preferably an anode. In addition, thefirst electrode preferably contains a material having a work function of3.5 eV to 5.5 eV.

Since the above-described light emitting element of the invention canreduce a drive voltage, a light emitting device having the lightemitting element of the invention can also reduce power consumption. Inaddition, since the light emitting element of the invention can bemanufactured at low cost, the light emitting device having the lightemitting element of the invention can also be manufactured at low cost.Therefore, the light emitting device using the light emitting element ofthe invention is also included in the present invention.

Note that the light emitting device in this specification refers to animage display device or an illuminator using a light emitting element.Further, the light emitting device includes all of the followingmodules: a module having a light emitting element provided with aconnector such as an anisotropic conductive film (FPC: Flexible PrintedCircuit), a TAB (Tape Automated Bonding) tape, or a TCP (Tape CarrierPackage); a module having a TAB tape or a TCP provided with a printedwiring board at the end thereof; and a module having an IC (IntegratedCircuit) directly mounted on a light emitting element by a COG (Chip OnGlass) method.

By carrying out the invention, a material which does not substantiallyhave a hole injection barrier from an electrode can be provided. Inaddition, a material which does not substantially have a hole injectionbarrier with various electrodes can be provided. Further, a materialhaving these characteristics and high heat resistance can be provided.

In addition, by using the material, a light emitting element and a lightemitting device with a low drive voltage can be provided. Further, aninexpensive light emitting element and an inexpensive light emittingdevice can be provided by using a combination of the material andgeneral-purpose metal.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show contact between a composite material of the presentinvention and an electrode.

FIG. 2 shows contact between a conventional organic compound and anelectrode.

FIG. 3 shows a structure of a light emitting element of the presentinvention.

FIG. 4 shows a structure of a light emitting element of the presentinvention.

FIGS. 5A to 5C show a structure of a light emitting device of thepresent invention.

FIGS. 6A to 6G show electric appliances using light emitting devices ofthe present invention.

FIG. 7 shows an absorption spectrum of a composite material of thepresent invention.

FIGS. 8A and 8B show current-voltage characteristics of ComparativeExample.

FIG. 9 shows an Arrhenius plot when Comparative Example is assumed to bea Schottky injection mechanism.

FIGS. 10A and 10B show current-voltage characteristics of a compositematerial of the present invention.

FIG. 11 shows an Arrhenius plot when current-voltage characteristics ofa composite material of the present invention are assumed to be aSchottky injection mechanism.

FIG. 12 is a diagram in which current-voltage characteristics of acomposite material of the present invention is fitted by Formula (1).

FIG. 13 shows an Arrhenius plot when current-voltage characteristics ofa composite material of the present invention are assumed to followFormula (1).

FIG. 14 shows an absorption spectrum of a composite material of thepresent invention.

FIGS. 15A and 15B show current-voltage characteristics of ComparativeExample.

FIG. 16 shows an Arrhenius plot when Comparative Example is assumed tobe a Schottky injection mechanism.

FIGS. 17A and 17B show current-voltage characteristics of a compositematerial of the present invention.

FIG. 18 shows an Arrhenius plot when current-voltage characteristics ofa composite material of the present invention are assumed to be aSchottky injection mechanism.

FIG. 19 is a diagram in which current-voltage characteristics of acomposite material of the present invention is fitted by Formula (1).

FIG. 20 shows an Arrhenius plot when current-voltage characteristics ofa composite material of the present invention are assumed to followFormula (1).

FIG. 21 shows an absorption spectrum of a composite material of thepresent invention.

FIGS. 22A and 22B show current-voltage characteristics of ComparativeExample.

FIG. 23 shows an Arrhenius plot when Comparative Example is assumed tobe a Schottky injection mechanism.

FIGS. 24A and 24B show current-voltage characteristics of a compositematerial of the present invention.

FIG. 25 shows an Arrhenius plot when current-voltage characteristics ofa composite material of the present invention are assumed to be aSchottky injection mechanism.

FIG. 26 is a diagram in which current-voltage characteristics of acomposite material of the present invention is fitted by Formula (1).

FIG. 27 shows an Arrhenius plot when current-voltage characteristics ofa composite material of the present invention are assumed to followFormula (1).

BEST MODE FOR CARRYING OUT THE INVENTION

Before explaining modes of a composite material of the presentinvention, the case of using a conventional organic compound is given asan example to explain its problem.

FIG. 2 is a schematic diagram of an energy level for explaining the casewhere a hole 230 is injected from an electrode 200 into an organiccompound 210. In the diagram, reference numeral 201 denotes a Fermilevel of the electrode 200; 211, a HOMO level of the organic compound210; and 212, a LUMO level of the organic compound 210. Since holes aregenerally injected into a HOMO level, reference numeral 220 in thediagram corresponds to a Schottky barrier for the hole 230. Thus, inorder to lower the Schottky barrier 220, it is necessary to lower alocation of the Fermi level 201 of the electrode (in other words, toincrease a work function) or to raise a location of the HOMO level 211of the organic compound. However, it is relatively difficult to raisethe HOMO level of the organic compound which can accept and transportholes to more than −5 eV, and it is difficult to substantially eliminatethe Schottky barrier 220 unless an electrode having a work functionhigher than 5 eV.

At this time, the amount of current when the hole injected from theelectrode 200 into the organic compound 210 flows is controlled by aformula of current density Js of a Schottky injection mechanismexpressed by the following Formula (2). In other words, such a device inwhich the electrode 200 is combined with the organic compound 210 is notcontrolled by conductivity of the bulk, but the device is controlled byinjection.

$\begin{matrix}{J_{s} = {A*T^{2}\exp\left\{ {- \frac{\phi_{B} - {q\sqrt{{qV}/\left( {4\pi\; ɛ_{i}d} \right)}}}{kT}} \right\}}} & (2)\end{matrix}$

(T denotes a temperature; φ_(B), a Schottky barrier; q, the amount ofelementary charge; V, a voltage; ∈_(i), a dielectric constant of theorganic compound; d, inter-electrode distance; and k, Boltzmannconstant.)

Thus, since flowing current greatly depends on a Schottky barrier φ_(B)in the conventional organic compound, there is significant limitation onthe kind of materials of an electrode and an organic compound in thedevice in which the electrode is combined with the organic compound.

It is the composite material of the invention that overcomes thisproblem. Hereinafter, one mode of the composite material of theinvention is first explained as Embodiment Mode 1.

Embodiment Mode 1

One mode of the composite material of the invention is a structureincluding an organic compound and an inorganic compound which shows anelectron-accepting property to the organic compound. At this time, thecomposite material is made to have a property of an impuritysemiconductor (p-type) having a high impurity concentration by mixing alarge amount of inorganic compounds showing an electron-acceptingproperty. In other words, a conduction mechanism is band conduction.

In the case of having a property of the impurity semiconductor (p-type)having a high impurity concentration, a schematic diagram of an energylevel when a hole is injected from an electrode 100 into a compositematerial 110 of the invention is as shown in FIGS. 1A and 1B. FIG. 1Aschematically shows a state before the electrode 100 has contact withthe composite material 110 of the invention. Reference numeral 101denotes a Fermi level of the electrode 100; 111, an upper limit of avalence band of the composite material 110; 112, a lower limit of aconduction band of the composite material 110; and 113 is a Fermi levelof the composite material 110.

When the electrode 100 has contact with the composite material 110,electrons move so that the Fermi levels correspond to each other.Accordingly, since the electron can pass through a barrier by atunneling effect as shown in FIG. 1B, a Schottky barrier (here, a holeinjection barrier) substantially disappears.

Meanwhile, ohmic current flows through the impurity semiconductor inwhich the Schottky barrier has disappeared, and current-voltagecharacteristics thereof follow Ohm's law. Consequently, current densityJ_(oh) thereof can be expressed by the following Formula (3).J _(oh) =σE=(σ/d)V  (3)

(σ denotes a conductivity; E, a field intensity; d, inter-electrodedistance; and V, a voltage.)

At this time, the conductivity σ of the impurity semiconductor hastemperature dependency such that an Arrhenius plot becomes linear asexpressed by the following Formula (4).

$\begin{matrix}{\sigma = {\sigma_{0}{\exp\left( \frac{- \phi_{a}}{2{kT}} \right)}}} & (4)\end{matrix}$

(φ_(a) denotes activation energy for carrier generation; T, atemperature; k, Boltzmann constant; and φ₀, a material-specificconstant.)

Consequently, current density J_(oh) of the impurity semiconductor inwhich the Schottky barrier has disappeared is expressed by the followingFormula (5) according to Formulae (3) and (4).

$\begin{matrix}{J_{oh} = {\left( {\sigma_{0}/d} \right)\left\{ {\exp\left( \frac{- \phi_{a}}{2{kT}} \right)} \right\} V}} & (5)\end{matrix}$

In the case of the composite material of the invention, band-conductioncurrent expressed by Formula (5) can flow as a result of the substantialdisappearance of the Schottky barrier. Since this characteristic can beobtained almost independently of the location of the HOMO level of theorganic compound in the composite material, various organic compoundscan be applied to the composite material of the invention.

And, not only that, the present inventor has found that a term of atrap-charge limited current is added to current flowing through thecomposite material of the invention. The trap-charge limited current isa kind of space-charge limited current, and is current peculiar to athin film at the time of conducting carriers injected from outside, andhopping conduction among molecules of the organic compound correspondsto it. Current density J_(t) thereof is expressed by exponentiation ofvoltage as in the following Formula (6). Note that B is affected bymobility, a dielectric constant, the number of traps, inter-electrodedistance, or the like; therefore, it can be said that B is a parameterdetermined by the inter-electrode distance or the kind of the compositematerial.J _(t) =BV ^(n)  (6)

(B denotes a parameter determined by inter-electrode distance and thekind of the composite material; n, an integer of 2 to 10; and V, avoltage.)

In other words, current density J=J_(oh)+J_(t) of the current flowingthrough the composite material of the invention is expressed by thefollowing Formula 1 according to Formulae (5) and (6) (note that it isset that σ₀/d=A).

$\begin{matrix}{J = {{\left\{ {A\;{\exp\left( \frac{- \phi_{a}}{2{kT}} \right)}} \right\} V} + {BV}^{n}}} & (1)\end{matrix}$

(J denotes a current density; V, a voltage; φ_(a), activation energy forcarrier generation in the composite material; k, Boltzmann constant; T,a temperature; A and B are parameters determined by a distance betweenthe pair of electrodes d, the amount of elementary charge q, a mobilityμ determined by the kind of the composite material, a dielectricconstant ∈, the number of traps per unit volume Nt, the number of LUMOlevels per volume of the organic compound in the composite materialN_(LUMO); and n, an integer of 2 to 10.)

A factor in causing the current-voltage characteristics of the compositematerial of the invention to follow Formula (1) is that the compositematerial of the invention has a property of an impurity semiconductorhaving high impurity concentration (which forms an ohmic contact with anelectrode and follows Ohm's law) and also a property of the used organiccompound (with which trap-charge limited current flows). In other words,one feature of the composite material of the invention showingcurrent-voltage characteristics expressed by Formula (1) is to have bothband conduction using carrier generation (here, hole generation) byadding an electron-accepting material to the organic compound andhopping conduction between organic compounds when the organic compoundtransports carriers injected from outside, and to be able to conduct alarge amount of current.

Further, it is understood that the above-described current-voltagecharacteristics can be obtained when a material having a work functionof 3.5 eV to 5.5 eV is used for the electrode. Formula (1) is a formulaassuming an ohmic contact, which indicates that the composite materialof the invention can form an ohmic contact with the material having awork function of 3.5 eV to 5.5 eV.

Materials suitable for forming the composite material of the inventionin Embodiment Mode 1 are listed below, but the invention is not limitedto these.

The composite material of the invention in Embodiment Mode 1 contains anorganic compound and an inorganic compound, and the inorganic compoundshows an electron-accepting property to the organic compound. An effectsuch as improvement in heat resistance can also be obtained by using theinorganic compound. The inorganic compound is not limited particularly,but transition metal oxide is preferable. Titanium oxide, zirconiumoxide, hafnium oxide, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, orrhenium oxide is suitable.

Here, since the inorganic compound shows an electron-accepting property,holes are generated in the organic compound. Therefore, ahole-transporting organic compound is preferable as the organiccompound. As the hole-transporting organic compound, for example,phthalocyanine (abbr.: H₂Pc), copper phthalocyanine (abbr.: CuPc),vanadyl phthalocyanine (abbr.: VOPc),4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbr.: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbr.:MTDATA), 1,3,5-tris[N,N-di(m-tolyl)amino]benzene (abbr.: m-MTDAB),N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine(abbr.: TPD), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbr.:NPB), 4,4′-bis{N-[4-di(m-tolyl)amino]phenyl-N-phenylamino}biphenyl(abbr.: DNTPD), 4,4′-bis[N-(4-biphenylyl)-N-phenylamino]biphenyl (abbr.:BBPB), 4,4′,4″-tri(N-carbazolyl)triphenylamine (abbr.: TCTA), or thelike can be used; however, the organic compound is not limited to these.Among the above-mentioned compounds, an aromatic amine compound typifiedby TDATA, MTDATA, m-MTDAB, TPD, NPB, DNTPD, BBPB, TCTA, or the likeeasily generates holes, and is a compound group suitable for the organiccompound.

As a method for manufacturing the composite material of the invention, atechnique for evaporating both an organic compound and an inorganiccompound as described above by resistance heating for co-evaporation canbe given. In addition, co-evaporation may be performed by evaporatingthe organic compound by resistance heating and evaporating the inorganiccompound by an electron beam (EB). Further, a technique forsimultaneously depositing both the organic compound and the inorganiccompound by evaporating the organic compound by resistance heating andby sputtering the inorganic compound can also be given. Alternatively,deposition may be performed by a wet method.

Embodiment Mode 2

Embodiment Mode 2 explains a current-excitation light emitting elementusing such a composite material of the present invention as described inEmbodiment Mode 1. A typical element structure is shown in FIG. 3. Thelight emitting element of the invention includes a first layer 311formed from such a composite material of the invention as described inEmbodiment Mode 1 and a second layer 312 containing a light emittingmaterial between a first electrode 301 and a second electrode 302, inwhich the first layer 311 is provided in contact with the firstelectrode 301.

Embodiment Mode 2 exemplifies an element in which current flows when apotential of the first electrode 301 is higher than that of the secondelectrode 302, and a hole 321 and an electron 322 are recombined witheach other in the second layer 312 to emit light. Thus, the firstelectrode 301 serves as an anode.

As described in Embodiment Mode 1, when a work function of a materialfor forming the first electrode 301 is approximately 3.5 eV to 5.5 eV,the first layer 311 forms an ohmic contact with the first electrode 301.Thus, a light emitting element with a low drive voltage can bemanufactured.

In addition, a material having a work function ranging from 3.5 eV to5.5 eV can be used for the first electrode 301. Specifically, atransparent electrode of indium tin oxide (hereinafter referred to asITO), indium tin oxide to which silicon is added (hereinafter referredto as ITSO), or the like, titanium, molybdenum, tungsten, nickel, gold,platinum, silver, aluminum, an alloy thereof, or the like can be used.In particular, titanium, molybdenum, aluminum, or an alloy thereof isgeneral-purpose metal often used for a wiring or the like, and when usedfor the first electrode 301, an inexpensive light emitting element canbe provided. It is one feature of the invention that metal normallyhaving difficulty in hole injection, such as aluminum (having a workfunction of approximately 4 eV), can be used for the first electrode301.

The second electrode 302 can be formed from the same material as thefirst electrode 301. Note that metal having a low work function, such aslithium, magnesium, calcium, or barium or an alloy thereof may be used.

Note that either or both the first electrode 301 and the secondelectrode 302 may be transparent to extract light from the lightemitting element. In addition, a substrate for supporting the lightemitting element may be provided either on the first electrode 301 sideor on the second electrode 302 side.

Subsequently, the second layer 312 is explained. The second layer 312 isa layer having a light-emitting function, and may contain at least alight emitting organic compound. In addition, the second layer 312 canbe appropriately combined with a hole transporting material, an electrontransporting material, or an electron injecting material. The secondlayer 312 may be a single layer of only a light emitting layercontaining a light emitting organic compound, or it may be a multilayercombined with a hole transporting layer, an electron transporting layer,an electron injecting layer, or the like.

As the light emitting organic compound, for example,9,10-di(2-naphthyl)anthracene (abbr.: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbr.: t-BuDNA),4,4′-bis(2,2-diphenylvinyl)biphenyl (abbr.: DPVBi), Coumarin 30,Coumarin 6, Coumarin 545, Coumarin 545T, perylene, rubrene,periflanthene, 2,5,8,11-tetra(tert-butyl)perylene (abbr.: TBP),9,10-diphenylanthracene (abbr.: DPA), 5,12-diphenyltetracene,4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran(abbr.: DCM1),4-(dicyanomethylene)-2-methyl-6-[2-(julolidine-9-yl)ethenyl]-4H-pyran(abbr.: DCM2),4-(dicyanomethylene)-2,6-bis[p-(dimethylamino)styryl]-4H-pyran (abbr.:BisDCM), or the like can be given. In addition, a compound which canemit phosphorescence can be used, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′](picolinato)iridium (abbr.:FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C²′}(picolinato)iridium(abbr.: Ir(CF₃ppy)₂(pic)), tris(2-phenylpyridinato-N,C²′)iridium (abbr.:Ir(ppy)₃), (acetylacetonato)bis(2-phenylpyridinato-N,C²′)iridium (abbr.:Ir(ppy)₂(acac)),(acetylacetonato)bis[2-(2′-thienyl)pyridinato-N,C³′]iridium (abbr.:Ir(thp)₂(acac)), (acetylacetonato)bis(2-phenylquinolinato-N,C²′)iridium(abbr.: Ir(pq)₂(acac)), or(acetylacetonato)bis[2-(2′-benzothienyl)pyridinato-N,C³′]iridium (abbr.:Ir(btp)₂(acac)).

As the hole transporting material which can be used in combination withthe light emitting organic compound, for example, thepreviously-described TDATA, MTDATA, m-MTDAB, TPD, NPB, DNTPD, BBPB,TCTA, or the like can be given. As the electron transporting material,for example, tris(8-quinolinolato)aluminum (abbr.: Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbr.: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbr.: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbr.: BAlq),bis[2-(2′-hydroxyphenyl)benzoxazolato]zinc (abbr.: Zn(BOX)₂),bis[2-(2′-hydroxyphenyl)benzothiazolato]zinc (abbr.: Zn(BTZ)₂),bathophenanthroline (abbr.: BPhen), bathocuproin (abbr.: BCP),2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbr.: PBD),1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (abbr.:OXD-7), 2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbr.: TPBI),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbr.:TAZ),3-(4-biphenylyl)-4-(4-ethylphenyl)-5-(4-tert-butylphenyl)-1,2,4-triazole(abbr.: p-EtTAZ), or the like can be given; however, the electrontransporting material is not limited thereto. As the electron injectingmaterial, an ultrathin film of an insulator, for example, alkali metalhalide such as LiF or CsF, alkaline earth metal halide such as CaF₂,alkali metal oxide such as Li₂O, or the like is often used besides theabove-described electron transporting material. Further, an alkali metalcomplex such as lithium acetylacetonate (abbr.: Li(acac)) or8-quinolinolato-lithium (abbr.: Liq) is also effective. Moreover, amaterial in which the above-mentioned electron transporting material ismixed with metal having a low work function, such as Mg, Li, or Cs, byco-evaporation or the like can also be used.

Note that the light emitting organic compound may be dispersed in theabove-mentioned hole transporting material or electron transportingmaterial, 4,4′-di(N-carbazolyl)biphenyl (abbr.: CBP), or the like.

Since the first layer 311 is formed from the composite material of theinvention, the first layer 311 can be formed by such a technique asdescribed in Embodiment Mode 1. In addition, the second layer 312 can beformed by an evaporation method using resistance heating or a wet methodsuch as spin coating, ink-jetting, or printing. Similarly, the firstelectrode 301 and the second electrode 302 can also be formed by anevaporation method using resistance heating, an EB evaporation method, asputtering method, a wet method, or the like.

Embodiment Mode 3

Embodiment Mode 3 explains one mode of the light emitting element of thepresent invention, which is different from that in Embodiment Mode 1. Anelement structure is shown in FIG. 4. A light emitting element ofEmbodiment Mode 3 includes a first layer 411 formed from the compositematerial of the invention as described in Embodiment Mode 1 and a secondlayer 412 containing a light emitting material between a first electrode401 and a second electrode 402, in which the first layer 411 is providedin contact with the first electrode 401. In addition, a third layer 413which generates electrons is provided between the first layer 411 andthe second layer 412.

In Embodiment Mode 3, current flows when a potential of the firstelectrode 401 is lower than that of the second electrode 402. At thistime, an electron 422 injected from the third layer 413 into the secondlayer 412 and a hole 423 injected from the second electrode 402 arerecombined with each other in the second layer 412 to emit light. On theother hand, the first layer 411 using the composite material of theinvention transfers a hole 421 generated in the vicinity of an interfacebetween the first layer 411 and the third layer 413 to the firstelectrode 401.

As described in Embodiment Mode 1, when a work function of a materialfor forming the first electrode 401 is approximately 3.5 eV to 5.5 eV,the first layer 411 forms an ohmic contact with the first electrode 401.Thus, a light emitting element with a low drive voltage can bemanufactured.

In addition, a material having a work function ranging from 3.5 eV to5.5 eV can be used for the first electrode 401. Specifically, atransparent electrode of indium tin oxide (ITO), indium tin oxide towhich silicon is added (ITSO), or the like, titanium, molybdenum,tungsten, nickel, gold, platinum, silver, aluminum, an alloy thereof, orthe like can be used. In particular, titanium, molybdenum, aluminum, oran alloy thereof is general-purpose metal often used for a wiring or thelike, and when used for the first electrode 401, an inexpensive lightemitting element can be provided. The second electrode 402 can also beformed from the same material as the first electrode 401.

Note that either or both the first electrode 401 and the secondelectrode 402 may be transparent to extract light from the lightemitting element. In addition, a substrate for supporting the lightemitting element may be provided either on the first electrode 401 sideor on the second electrode 402 side.

The second layer 412 is a layer having a light-emitting function, andmay contain at least a light emitting organic compound. As a structurethereof, a similar structure to that of the second layer described inEmbodiment Mode 2 can be employed.

The third layer is not particularly limited as long as it can generateelectrons. Specifically, the third layer may include a layer containingan electron transporting organic compound and a material showing anelectron donating property to the organic compound. As the electrontransporting organic compound, the previously-described Alq₃, Almq₃,BeBq₂, BAlq, Zn(BOX)₂, Zn(BTZ)₂, BPhen, BCP, PBD, OXD-7, TPBI, TAZ,p-EtTAZ, or the like can be used. As the material showing an electrondonating property, alkali metal or alkaline earth metal such as lithium,magnesium, calcium, or barium or an alloy thereof can be given. Analkali metal compound or an alkaline earth metal compound such aslithium oxide, barium oxide, lithium nitride, magnesium nitride, orcalcium nitride can also be used.

Note that a hole injecting layer may be provided between the secondlayer 412 and the second electrode 402. As a material which can be usedfor the hole injecting layer, the composite material of the invention asdescribed in Embodiment Mode 1 may be employed as well as H₂Pc, CuPc, orVOPc.

Since the first layer 411 is formed from the composite material of theinvention, the first layer 411 can be formed by such a technique asdescribed in Embodiment Mode 1. In addition, the second layer 412 or thethird layer 413 can be formed by an evaporation method by resistanceheating or a wet method such as spin coating, ink-jetting, or printing.Similarly, the first electrode 401 and the second electrode 402 can alsobe formed by an evaporation method using resistance heating, an EBevaporation method, a sputtering method, a wet method, or the like.

Embodiment Mode 4

A structure of the light emitting device of the present invention isexplained with reference to FIGS. 5A to 5C. Note that FIG. 5A is a topview of the light emitting device; FIG. 5B is a cross-sectional detailview taken along line A-A′ in FIG. 5A; and FIG. 5C is a cross-sectionalstructure view of the light emitting device. In FIG. 5A, a source sidedriver circuit 501, a pixel portion 502, and a gate side driver circuit503 are indicated by dotted lines. In addition, a sealing substrate 504is fixed to a substrate 510, over which a TFT and a light emittingelement are formed, with a sealant 505. The source side driver circuit501, the pixel portion 502, and the gate side driver circuit 503 aresealed between the substrate 510 and the sealing substrate 504. An innerregion surrounded by the sealant 505 is filled with a filler 506. Thefiller 506 may be an inert gas or a solid such as a resin. Note that aresin material having low water vapor permeability is preferably used asthe sealant 505 and the filler 506.

A connection wiring 507 transmits a signal inputted to the source sidedriver circuit 501 and the gate side driver circuit 503, and is arrangedso as to be extended to an end portion of the substrate 510. A flexibleprinted circuit board (FPC) 508 to be connected to an external circuitis connected to an end portion of the connection wiring 507. Aproportion of the pixel portion 502 to the substrate 510 can beincreased by forming a seal pattern with the sealant 505 so as tooverlap this connection portion. In other words, a width of a so-calledframe region where a driver circuit or a connection region such as anFPC is formed over the substrate 510 can be reduced.

FIG. 5C is a cross-sectional structure view of the light emittingdevice. An element formation region 808 for the source side drivercircuit, the pixel portion, the gate side driver circuit, or the likeover the substrate 510 is sealed with the filler 506 and the sealingsubstrate 504. The flexible printed circuit board (FPC) 508 is connectedto a circuit board 807 arranged on the substrate 510 side or the sealingsubstrate 504 side. The circuit board 807 is provided with a controlcircuit for controlling this light emitting device, a power supplycircuit, or the like. Downsizing of this module can be attempted bybending and connecting the flexible printed circuit board (FPC) 508 tothe circuit board 807 arranged on the substrate 810 side or the sealingsubstrate 504 side. When such a module structure is applied to a smallelectronic device such as a cellular phone or an electronic organizer,the device can be attempted to be miniaturized.

Subsequently, a cross-sectional structure is explained with reference toFIG. 5B. A driver circuit portion and a pixel portion are formed overthe substrate 510, but here, the source side driver circuit 501 which isa driver circuit portion and the pixel portion 502 are shown.

Note that a CMOS circuit in which an n-channel TFT 523 is combined witha p-channel TFT 524 is formed as the source side driver circuit 501. Inaddition, a TFT for forming a driver circuit may be formed with awell-known CMOS circuit, PMOS circuit, or NMOS circuit. In thisembodiment mode, a driver-integrated type in which a driver circuit isformed over a substrate is shown; however, the driver circuit does notnecessarily have to be formed over the substrate. The driver circuit canbe formed outside the substrate instead of being formed over thesubstrate.

The pixel portion 502 is formed with a plurality of pixels eachincluding a switching TFT 511, a current control TFT 512, and a firstelectrode 513 electrically connected to a drain of the current controlTFT 512. Note that an insulator 514 is formed to cover an end portion ofthe first electrode 513. Here, the insulator 514 is formed using apositive-type photosensitive acrylic resin film.

In addition, the insulator 514 is formed to have a curved surface with acurvature at an upper end portion or a lower end portion in order tomake coverage favorable. In the case of using, for example, apositive-type photosensitive acrylic as a material of the insulator 514,the insulator 514 is preferably formed to have a curved surface with acurvature radius (0.2 μm to 3 μm) only at an upper end portion. Further,as the insulator 514, either a photosensitive negative type whichbecomes insoluble in an etchant by light or a photosensitive positivetype which becomes soluble in an etchant by light can be used.

A layer 515 and a second electrode 516 are formed over the firstelectrode 513, which constitutes part of a light emitting element 517.The light emitting element 517 may employ such a structure of the lightemitting element as described in Embodiment Mode 2 or 3. Thus, the layer515 includes at least the first layer and the second layer described inEmbodiment Mode 2 or 3, and the first layer is provided in contact withthe first electrode 513. The first electrode 513 and the secondelectrode 516 may also employ the structure described previously inEmbodiment 2 or 3.

Although the connection wiring 507 and the first electrode 513 are eachformed from different materials in Embodiment Mode 4, they may be formedfrom the same material. In other words, the connection wiring 507 can beused as the first electrode 513 without any change. Therefore, thenumber of steps can be reduced, which leads to cost reduction. This isan advantage resulting from the fact that the first layer containing thecomposite material of the invention has capability of forming an ohmiccontact with the first electrode 513.

By attaching the sealing substrate 504 to the substrate 510 with thesealant 505, a structure can be formed in which the light emittingelement 517 is provided in the region surrounded by the substrate 510,the sealing substrate 504, and the sealant 505. Note that the regionsurrounded by the substrate 510, the sealing substrate 504, and thesealant 505 may be filled with the sealant 505 as well as an inert gas(nitrogen, argon, or the like) as the filler 506.

Note that an epoxy-based resin is preferably used as the sealant 505.This material is preferably a material which is permeated by as littlemoisture and oxygen as possible. In addition to a glass substrate or aquartz substrate, a plastic substrate formed from FRP(Fiberglass-Reinforced Plastics), RVF (polyvinylfluoride), myler,polyester, acrylic, or the like can be used for the sealing substrate504.

As described above, the light emitting device using the light emittingelement of the invention can be obtained.

Embodiment Mode 5

This embodiment mode explains various electric appliances completed byutilizing the light emitting device using the light emitting element ofthe present invention.

Examples of electric appliances manufactured by utilizing the lightemitting device with the light emitting of the invention can be given asfollows: a camera such as a video camera or a digital camera, a goggletype display (head-mounted display), a navigation system, a soundreproducing device (car audio, an audio component, or the like), apersonal computer, a game machine, a portable information terminal (amobile computer, a cellular phone, a portable game machine, anelectronic book, or the like), an image reproducing device provided witha recording medium (specifically, a device which can reproduce therecording medium such as a digital versatile disc (DVD) and includes adisplay device capable of displaying images thereof), and the like.Specific examples thereof are shown in FIGS. 6A to 6G.

FIG. 6A shows a display device, which includes a chassis 6101, a support6102, a display portion 6103, a speaker portion 6104, a video inputterminal 6105, and the like. The display device is manufactured by usingthe light emitting device having the light emitting element of theinvention for the display portion 6103. Note that the display deviceincludes all devices used for displaying information, for example, for apersonal computer, for TV broadcast reception, for advertisementdisplay, and the like.

FIG. 6B shows a notebook-type personal computer, which includes a mainbody 6201, a chassis 6202, a display portion 6203, a keyboard 6204, anexternal connection port 6205, a pointing mouse 6206, and the like. Thenotebook-type personal computer can be manufactured by using the lightemitting device having the light emitting element of the invention forthe display portion 6203.

FIG. 6C shows a mobile computer, which includes a main body 6301, adisplay portion 6302, a switch 6303, an operation key 6304, an infraredport 6305, and the like. The mobile computer can be manufactured byusing the light emitting device having the light emitting element of theinvention for the display portion 6302.

FIG. 6D shows a portable image reproducing device provided with arecording medium (specifically, a DVD reproducing device), whichincludes a main body 6401, a chassis 6402, a display portion A 6403, adisplay portion B 6404, a recording medium (a DVD or the like) readingportion 6405, an operation key 6406, a speaker portion 6407, and thelike. The display portion A 6403 mainly displays image information, andthe display portion B 6404 mainly displays character information. Theportable image reproducing device can be manufactured by using the lightemitting device having the light emitting element of the invention forthe display portion A 6403 and the display portion B 6404. Note that theimage reproducing device provided with a recording medium includes ahome-use game machine and the like.

FIG. 6E shows a goggle type display (head-mounted display), whichincludes a main body 6501, a display portion 6502, and an arm portion6503. The goggle type display can be manufactured by using the lightemitting device having the light emitting element of the invention forthe display portion 6502.

FIG. 6F shows a video camera, which includes a main body 6601, a displayportion 6602, a chassis 6603, an external connection port 6604, a remotecontrol receiving portion 6605, an image receiving portion 6606, abattery 6607, an audio input portion 6608, operation keys 6609, an eyepiece portion 6610, and the like. The video camera can be manufacturedby using the light emitting device having the light emitting element ofthe invention for the display portion 6602.

FIG. 6G shows a cellular phone, which includes a main body 6701, achassis 6702, a display portion 6703, an audio input portion 6704, anaudio output portion 6705, an operation key 6706, an external connectionport 6707, an antenna 6708, and the like. The cellular phone can bemanufactured by using the light emitting device having the lightemitting element of the invention for the display portion 6703. Notethat power consumption of the cellular phone can be reduced when thedisplay portion 6703 displays white characters on a black background.

As described above, an application range of the light emitting devicehaving the light emitting element of the invention is so wide that thislight emitting device can be applied to electric appliances in variousfields.

Embodiment 1

Embodiment 1 exemplifies a composite material of the present inventionin which an organic compound is compounded with an inorganic compoundshowing an electron-accepting property to the organic compound. BBPBhaving a hole transporting property was used as the organic compound,and molybdenum oxide was used as the inorganic compound.

First, a glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus. Next, BBPB and molybdenum oxide (VI) wereseparately put in different resistance-heating evaporation sources, anda composite material of the invention in which BBPB was compounded withmolybdenum oxide was deposited under vacuum by a co-evaporation method.At this time, BBPB was evaporated at a deposition rate of 0.4 nm/s, andmolybdenum oxide of a quarter (weight ratio) of the amount of BBPB wasevaporated. Therefore, a molar ratio of BBPB to molybdenum oxide was1:1. Note that a thickness thereof was 50 nm.

A measurement result of an absorption spectrum of the BBPB-molybdenumoxide composite material which was deposited in this way is indicated byA. in FIG. 7. For comparison, absorption spectra of a film of only BBPB(B. in the diagram) and a film of only molybdenum oxide (C. in thediagram) are also shown.

As FIG. 7 shows, new absorption, which was not seen in each layer ofonly BBPB or molybdenum oxide, was observed in the composite material ofA. at around 500 nm, 800 nm, and 1500 nm. It is thought that this isbecause BBPB and molybdenum oxide transfer electrons, and molybdenumoxide accepts electrons from BBPB and holes are generated in BBPB.Accordingly, it is suggested that in the same manner as an impuritysemiconductor to which impurities are added at high concentration, thecomposite material of the invention can form an ohmic contact withvarious electrodes and can perform carrier transport like bandconduction.

On the other hand, absorption at around 350 nm, which is also seen inthe film of only BBPB (B.), is observed in the composite material (A.).This suggests that the composite material of the invention still has aproperty of BBPB, and can perform carrier transport by hoppingconduction (trap-charge limited current).

Embodiment 2

Embodiment 2 exemplifies current-voltage characteristics of thecomposite material of the present invention. First, current-voltagecharacteristics of the above-described film of only BBPB are exemplifiedfor comparison.

Comparative Example

First, a glass substrate, over which ITSO was deposited with a thicknessof 110 nm, was prepared. The periphery of ITSO was covered with aninsulating film so that a portion of the ITSO surface with a size of 2mm square was exposed.

Next, the glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that the side provided with ITSO faceddownward. Then, BBPB was put in a resistance-heating evaporation source,and BBPB was deposited under vacuum by a vacuum evaporation method. Athickness thereof was 200 nm. In addition, aluminum (Al) was depositedthereover with a thickness of 200 nm.

As to the laminated structure thus obtained in which ITSO, BBPB, and Alare sequentially laminated over the substrate, measurement results ofcurrent-voltage characteristics at −35° C., −20° C., −5° C., 10° C., 25°C., 40° C., 55° C., 70° C., 85° C., and 100° C. are shown in FIG. 8A.Note that the case where a potential of ITSO is higher than that of Alis regarded as forward bias.

Since current flows under forward bias as shown in FIG. 8A, it is foundthat holes are injected from ITSO. Further, since current does not flowand rectification is shown under reverse bias, it is suggested thatholes are not injected from Al.

Subsequently, the current-voltage characteristics obtained in FIG. 8Awere analyzed to see whether the current flowing in Comparative Examplewas actually controlled by a Schottky injection mechanism (in otherwords, whether it is controlled by injection). J_(s) at the time of V=0(referred to as J₀) in the above Formula (2) is expressed by thefollowing Formula (7).

$\begin{matrix}{J_{0} = {A*T^{2}{\exp\left( \frac{- \phi_{a}}{kT} \right)}}} & (7)\end{matrix}$

By transforming this formula, the following Formula (8) can be obtained.1n(J ₀ /T ²)=−φ_(B)/(kT)+1nA  (8)

Therefore, if a Schottky injection mechanism is dominant, J₀/T² issupposed to be linear when plotted on an Arrhenius plot. As shown inFIG. 8B, J₀ at respective temperatures can be obtained by replacing thehorizontal axis of the current-voltage characteristics obtained in FIG.8A with a square root of a voltage V, replacing the vertical axis with alogarithm of a current density J, and extrapolating a plot (solid linesin the diagram) at respective temperatures. The obtained values of J₀ atrespective temperatures are shown in the following Table 1.

TABLE 1 temperatures[° C.] J₀[mA/cm²] −35 1.0 × 10⁻⁵ −20 6.0 × 10⁻⁵ −53.0 × 10⁻⁴ 10 1.0 × 10⁻³ 25 3.4 × 10⁻³ 40 1.1 × 10⁻² 55 7.0 × 10⁻² 701.8 × 10⁻¹ 85 4.8 × 10⁻¹ 100 1.3

The values thus obtained of J₀ were used to make an Arrhenius plotaccording to Formula (8), and a result thereof is shown in FIG. 9. Sincethe Arrhenius plot shows linearity as shown in FIG. 9, it is suggestedthat hole injection from ITSO into BBPB is a Schottky injectionmechanism. In addition, it is also found that the Schottky injectioncontrols a current amount. Note that a Schottky barrier φ_(B) was foundfrom the slope of the plot in FIG. 9 to be 0.62 eV.

Example

Subsequently, current-voltage characteristics of the composite materialof the invention are exemplified. First, a glass substrate, over whichITSO was deposited with a thickness of 110 nm, was prepared. Theperiphery of ITSO was covered with an insulating film so that a portionof the ITSO surface with a size of 2 mm square was exposed.

Next, the glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that the side provided with ITSO faceddownward. Then, BBPB, molybdenum oxide (VI), and rubrene were separatelyput in different resistance-heating evaporation sources, and thecomposite material of the invention formed from BBPB, molybdenum oxide,and rubrene was deposited under vacuum by a co-evaporation method. Atthis time, BBPB was evaporated at a deposition rate of 0.2 nm/s and anadjustment was performed so that BBPB: molybdenum oxide: rubrene becomes2:0.75:0.02 (mass ratio). A thickness thereof was 200 nm to correspondto Comparative Example. Further, aluminum (Al) was deposited thereoverwith a thickness of 200 nm. Note that rubrene was added as a stabilizerof film quality, which is not necessarily required.

As to the laminated structure obtained thus in which ITSO, a mixed filmof BBPB, molybdenum oxide, and rubrene, and Al are sequentiallylaminated over the substrate, measurement results of current-voltagecharacteristics at −35° C., −20° C., −5° C., 10° C., 25° C., 40° C., 55°C., 70° C., 85° C., and 100° C. are shown in FIG. 10A. Not the casewhere a potential of ITSO is higher than that of Al is regarded asforward bias.

Since almost the same amount of current flows both under forward biasand reverse bias, which is different from the above Comparative Example(FIG. 8A), it is found that an equivalent amount of holes is injectedfrom both ITSO and Al. In addition, it is also found that a largeramount of current flows at low voltage as compared to the aboveComparative Example.

Subsequently, the current-voltage characteristics obtained in FIG. 10Awere analyzed to see whether current flowing in this example iscontrolled by a Schottky injection mechanism (in other words, whether itis controlled by injection). First, under forward bias (in other words,at the time of hole injection from ITSO), J₀ at respective temperatureswas found as in the above Comparative Example by replacing thehorizontal axis of the current-voltage characteristics obtained in FIG.10A with a square root of a voltage V, replacing the vertical axis witha logarithm of a current density J (see FIG. 10B), and extrapolating aplot (solid lines in the diagram) at respective temperatures.Subsequently, an Arrhenius plot was made according to Formula (8). Aresult thereof is shown in FIG. 11.

As shown in FIG. 11, it is found that in the case of using the compositematerial of the invention, an Arrhenius plot of J₀/T² is not linear.This suggests that a Schottky injection mechanism is not dominant as tohole injection from ITSO into the composite material of the invention.

Thus, the current-voltage characteristics of the composite material ofthe invention were analyzed to see whether they followed Formula (1)described in Embodiment Mode 1. When it is set that Aexp (−φ_(a)/(2kT))=A′ in Formula (1), Formula (1) can be expressed as the followingFormula (9).J=A′V+BV ^(n)  (9)

However, A′ is expressed by the following Formula (10)

$\begin{matrix}{A^{\prime} = {A\;{\exp\left( \frac{- \phi_{a}}{2{kT}} \right)}}} & (10)\end{matrix}$

A result of fitting the plot under forward bias in FIG. 10A by Formula(9) is shown in FIG. 12. Broken lines in the diagram show the fitting byFormula (9) in the case of n=5. As FIG. 12 shows, it is found that thefitting is performed with extremely precision. Values of A′ found bythis fitting at respective temperatures are shown in the following Table2.

TABLE 2 temperatures [° C.] A′[mA/cm²/V] −35 6.9 × 10  −20 1.1 × 10² −51.6 × 10² 10 2.2 × 10² 25 2.8 × 10² 40 3.5 × 10² 55 4.7 × 10² 70 5.5 ×10² 85 5.3 × 10² 100 7.0 × 10²

Here, since the following Formula (11) can be obtained according toFormula (10), A′ is supposed to be linear when plotted on an Arrheniusplot. A result thereof is shown in FIG. 13.

$\begin{matrix}{{\ln\; A^{\prime}} = {\frac{- \phi_{a}}{2{kT}} + {\ln\; A}}} & (11)\end{matrix}$

Since the Arrhenius plot shows linearity as shown in FIG. 13, it isfound that an ohmic contact is formed as to hole injection from ITSOinto the composite material of the invention and a current amountthereof follows Formula (1). Note that activation energy φ_(a) at thistime is 0.26 eV.

Since almost the same amount of current as that under forward bias alsoflows under reverse bias, it is found that an ohmic contact is formed asto hole injection from Al into the composite material of the inventionand a current amount thereof follows Formula (1).

Note that a work function of ITSO is 4.89 eV and that of Al isapproximately 4 eV (each of which is measured using a photoelectronspectrometer AC-2 (manufactured by Riken Keiki Co., Ltd.)). According tothe above, it is found that measured current-voltage characteristics ofthe composite material of the invention sandwiched between electrodeseach having a work function of 3.5 eV to 5.5 eV follow Formula (1).

In addition, following Formula (1) allows the composite material of theinvention to form an ohmic with the electrode and in addition, toconduct a large amount of current. Thus, a light emitting element inwhich the composite material of the invention is provided in contactwith the electrode can reduce a drive voltage. In addition,general-purpose metal such as aluminum can be used for an anode.

Embodiment 3

Embodiment 3 exemplifies a composite material of the invention in whichan organic compound is compounded with an inorganic compound showing anelectron-accepting property to the organic compound. NPB having a holetransporting property was used as the organic compound, and molybdenumoxide was used as the inorganic compound.

First, a composite material of the invention in which NPB was compoundedwith molybdenum oxide was deposited by a co-evaporation method. At thistime, NPB was evaporated at a deposition rate of 0.4 nm/s, andmolybdenum oxide of a quarter (weight ratio) of the amount of NPB wasevaporated. Therefore, a molar ratio of NPB to molybdenum oxide was 1:1.Note that a thickness thereof was 50 nm.

A measurement result of an absorption spectrum of the composite materialof NPB and molybdenum oxide which was deposited in this way is indicatedby A. in FIG. 14. For comparison, an absorption spectrum of a film ofonly NPB (B. in the diagram) is also shown. An absorption spectrum of afilm of only molybdenum oxide is omitted here since it is shown in FIG.7 in Embodiment 1.

As FIG. 14 shows, new absorption, which was not seen in each layer ofonly NPB or molybdenum oxide, was observed in the composite material ofA. at around 500 nm, 800 nm, and 1400 nm. It is thought that this isbecause NPB and molybdenum oxide transfer electrons, and molybdenumoxide accepts electrons from NPB and holes are generated in NPB.Accordingly, it is suggested that in the same manner as an impuritysemiconductor doped with impurities at high concentration, the compositematerial of the invention can form an ohmic contact with variouselectrodes and can perform carrier transport like band conduction.

On the other hand, absorption at around 350 nm, which was also seen inthe film of only NPB (B.), was observed in the composite material (A.).This suggests that the composite material of the invention still has aproperty of NPB, and can perform carrier transport by hopping conduction(trap-charge limited current).

Embodiment 4

Embodiment 4 exemplifies current-voltage characteristics of thecomposite material of the invention. First, current-voltagecharacteristics of the above-described film of only NPB are exemplifiedfor comparison.

Comparative Example

First, a glass substrate, over which ITSO was deposited with a thicknessof 110 nm, was prepared. The periphery of ITSO was covered with aninsulating film so that a portion of the ITSO surface with a size of 2mm square was exposed.

Next, the glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that the side provided with ITSO faceddownward. Then, NPB was put in a resistance-heating evaporation source,and NPB was deposited under vacuum by a vacuum evaporation method. Athickness thereof was 200 nm. In addition, aluminum (Al) was depositedthereover with a thickness of 200 nm.

As to the laminated structure thus obtained in which ITSO, NPB, and Alare sequentially laminated over the substrate, measurement results ofcurrent-voltage characteristics at −35° C., −20° C., −5° C., 10° C., 25°C., 40° C., 55° C., 70° C., and 85° C. are shown in FIG. 15A (data at100° C. was not obtained since a glass transition point of NPB isslightly less than 100° C.). Note that the case where a potential ofITSO is higher than that of Al is regarded as forward bias.

Since current flows under forward bias as shown in FIG. 15A, it is foundthat holes are injected from ITSO. Further, since current does not flowand rectification is shown under reverse bias, it is suggested thatholes are not injected from Al.

Subsequently, the current-voltage characteristics obtained in FIG. 15Awere analyzed to see whether current flowing in Comparative Example wasactually controlled by a Schottky injection mechanism (in other words,whether it is controlled by injection). A method of analysis is the sameas that described in Embodiment 2. The y-intersect (J₀) at respectivetemperatures can be obtained by replacing the horizontal axis of thecurrent-voltage characteristics obtained in FIG. 15A with a square rootof a voltage V, replacing the vertical axis with a logarithm of acurrent density J (see FIG. 15B), and extrapolating a plot (solid linesin the diagram) at respective temperatures. It was determined if J₀ atrespective temperatures followed Formula (8).

The obtained values of J₀ at respective temperatures are shown in thefollowing Table 3. The values thus obtained of J₀ were used to make anArrhenius plot according to Formula (8), and a result thereof is shownin FIG. 16.

TABLE 3 temperatures [° C.] J₀[mA/cm²] −35 8.0 × 10⁻⁵ −20 9.0 × 10⁻⁵ −58.0 × 10⁻⁴ 10 1.9 × 10⁻³ 25 7.2 × 10⁻³ 40 2.2 × 10⁻² 55 6.2 × 10⁻² 701.4 × 10⁻¹ 85 2.8 × 10⁻¹

Since the Arrhenius plot shows linearity as shown in FIG. 16, it issuggested that hole injection from ITSO into NPB is a Schottky injectionmechanism. In addition, it is also found that the Schottky injectioncontrols a current amount. Note that a Schottky barrier φ_(B) is foundfrom the slope in FIG. 16 to be 0.49 eV.

Example

Subsequently, current-voltage characteristics of the composite materialof the invention are exemplified. First, a glass substrate, over whichITSO was deposited with a thickness of 110 nm, was prepared. Theperiphery of ITSO was covered with an insulating film so that a portionof the ITSO surface with a size of 2 mm square was exposed.

Next, the glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that the side provided with ITSO faceddownward. Then, NPB, molybdenum oxide (VI), and rubrene were separatelyput in different resistance-heating evaporation sources, and thecomposite material of the invention formed from NPB, molybdenum oxide,and rubrene was deposited under vacuum by a co-evaporation method. Atthis time, NPB was evaporated at a deposition rate of 0.2 nm/s and anadjustment was performed so that NPB: molybdenum oxide: rubrene becomes2:0.75:0.04 (mass ratio). A thickness thereof was 200 nm to correspondto Comparative Example. Further, aluminum (Al) was deposited thereoverwith a thickness of 200 nm. Note that rubrene was added as a stabilizerof film quality, which is not necessarily required.

As to the laminated structure obtained thus in which ITSO, a mixed filmof NPB, molybdenum oxide, and rubrene, and Al are sequentially laminatedover the substrate, measurement results of current-voltagecharacteristics at −35° C., −20° C., −5° C., 10° C., 25° C., 40° C., 55°C., 70° C., and 85° C. are shown in FIG. 17A. Note that the case where apotential of ITSO is higher than that of Al is regarded as forward bias.

Since almost the same amount of current flows both under forward biasand reverse bias, which is different from the above Comparative Example(FIG. 15A), it is found that an equivalent amount of holes is injectedfrom both ITSO and Al. In addition, it is also found that a largeramount of current flows at low voltage as compared to the aboveComparative Example.

Subsequently, the current-voltage characteristics obtained in FIG. 17Awere analyzed to see whether current flowing in this example wascontrolled by a Schottky injection mechanism (in other words, whether itis controlled by injection). First, under forward bias (in other words,at the time of hole injection from ITSO), J₀ was found as in the aboveComparative Example by replacing the horizontal axis of thecurrent-voltage characteristics obtained in FIG. 17A with a square rootof a voltage V, replacing the vertical axis with a logarithm of acurrent density J (see FIG. 17B), and extrapolating a plot (solid linesin the diagram) at respective temperatures. Subsequently, an Arrheniusplot was made according to Formula (8). A result thereof is shown inFIG. 18.

As shown in FIG. 18, it is found that in the case of using the compositematerial of the invention, an Arrhenius plot of J₀/T² is not linear.This suggests that a Schottky injection mechanism is not dominant as tohole injection from ITSO into the composite material of the invention.

Thus, the current-voltage characteristics of the composite material ofthe invention were analyzed to see whether they followed Formula (1)described in Embodiment Mode 1. A method of analysis is the same as thatdescribed in Embodiment 2. The plot under forward bias in FIG. 17A wasfitted by Formula (9) to find A′ at respective temperatures, and it wasdetermined if A′ followed Formula (11).

A result of fitting the plot under forward bias in FIG. 17A by Formula(9) is shown in FIG. 19. Broken lines in the diagram show the fitting byFormula (9) in the case of n=5. As FIG. 19 shows, it is found that thefitting is performed with extreme precision. Values of A′ found by thisfitting at respective temperatures are shown in the following Table 4.

TABLE 4 temperatures [° C.] A′[mA/cm²/V] −35 4.2 × 10  −20 7.4 × 10  −51.2 × 10² 10 1.7 × 10² 25 2.3 × 10² 40 3.0 × 10² 55 3.8 × 10² 70 4.6 ×10² 85 5.5 × 10²

A result of making an Arrhenius plot of A′ is shown in FIG. 20. Sincethe Arrhenius plot shows linearity as shown in FIG. 20, it is found thatan ohmic contact is formed as to hole injection from ITSO into thecomposite material of the invention and a current amount thereof followsFormula (1). Note that activation energy φ_(a) at this time is 0.31 eV.

Since almost the same amount of current as that under forward bias alsoflows under reverse bias, it is found that an ohmic contact is formed asto hole injection from Al into the composite material of the inventionand a current amount thereof follows Formula (1).

Note that a work function of ITSO is 4.89 eV and that of Al isapproximately 4 eV (each of which is measured using a photoelectronspectrometer AC-2 (manufactured by Riken Keiki Co., Ltd.)). According tothe above, it is found that measured current-voltage characteristics ofthe composite material of the invention sandwiched between electrodeseach having a work function of 3.5 eV to 5.5 eV follow Formula (1).

In addition, following Formula (1) allows the composite material of theinvention to have an ohmic with the electrode and in addition, toconduct a large amount of current. Thus, a light emitting element inwhich the composite material of the invention is provided in contactwith the electrode can reduce a drive voltage. In addition,general-purpose metal, which does not have a high work function, such asaluminum can be used for an anode.

Embodiment 5

Embodiment 5 exemplifies a composite material of the invention in whichan organic compound is compounded with an inorganic compound showing anelectron-accepting property to the organic compound. DNTPD having a holetransporting property was used as the organic compound, and molybdenumoxide was used as the inorganic compound.

First, a composite material of the invention in which DNTPD wascompounded with molybdenum oxide was deposited by a co-evaporationmethod as in Embodiment 1. At this time, DNTPD was evaporated at adeposition rate of 0.4 nm/s, and molybdenum oxide of a quarter (weightratio) of the amount of DNTPD was evaporated. Therefore, a molar ratioof DNTPD to molybdenum oxide was 1:1.5. Note that a thickness thereofwas 50 nm.

A measurement result of an absorption spectrum of the composite materialof DNTPD and molybdenum oxide which was deposited in this way isindicated by A. in FIG. 21. For comparison, an absorption spectrum of afilm of only DNTPD (B. in the diagram) is also shown. An absorptionspectrum of a film of only molybdenum oxide is omitted here since it isshown in FIG. 7 in Embodiment 1.

As FIG. 21 shows, new absorption, which was not seen in each layer ofonly DNTPD or molybdenum oxide, was observed in the composite materialof A. at around 900 nm. It is thought that this is because DNTPD andmolybdenum oxide transfer electrons, and molybdenum oxide acceptselectrons from DNTPD and holes are generated in DNTPD. Accordingly, itis suggested that in the same manner as an impurity semiconductor dopedwith impurities at high concentration, the composite material of theinvention can form an ohmic contact with various electrodes and canperform carrier transport like band conduction.

On the other hand, absorption at around 350 nm, which was also seen inthe film of only DNTPD (B.), was observed in the composite material(A.). This suggests that the composite material of the invention stillhas a property of DNTPD, and can perform carrier transport by hoppingconduction (trap-charge limited current).

Embodiment 6

Embodiment 6 exemplifies current-voltage characteristics of thecomposite material of the invention. First, current-voltagecharacteristics of the above-described film of only DNTPD areexemplified for comparison.

Comparative Example

First, a glass substrate, over which ITSO was deposited with a thicknessof 110 nm, was prepared. The periphery of ITSO was covered with aninsulating film so that a portion of the ITSO surface with a size of 2mm square was exposed.

Next, the glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that the side provided with ITSO faceddownward. Then, DNTPD was put in a resistance-heating evaporationsource, and DNTPD was deposited under vacuum by a vacuum evaporationmethod. A thickness thereof was 200 nm. In addition, aluminum (Al) wasdeposited thereover with a thickness of 200 nm.

As to the laminated structure thus obtained in which ITSO, DNTPD, and Alare sequentially laminated over the substrate, measurement results ofcurrent-voltage characteristics at −35° C., −20° C., −5° C., 10° C., 25°C., 40° C., 55° C., 70° C., and 85° C. are shown in FIG. 22A (data at100° C. was not obtained since a glass transition point of DNTPD isslightly less than 100° C.). Note that the case where a potential ofITSO is higher than that of Al is regarded as forward bias.

Since current flows under forward bias as shown in FIG. 22A, it is foundthat holes are injected from ITSO. Further, since current does not flowand rectification is shown under reverse bias, it is suggested thatholes are not injected from Al.

Subsequently, the current-voltage characteristics obtained in FIG. 22Awere analyzed to see whether current flowing in Comparative Example wasactually controlled by a Schottky injection mechanism (in other words,whether it is controlled by injection). A method of analysis is the sameas that described in Embodiment 2. The y-intersect (J₀) at respectivetemperatures was obtained by replacing the horizontal axis of thecurrent-voltage characteristics obtained in FIG. 22A with a square rootof a voltage V, replacing the vertical axis with a logarithm of acurrent density J (see FIG. 22B), and extrapolating a plot (solid linesin the diagram) at respective temperatures. It was determined if J₀ atrespective temperatures followed Formula (8).

The obtained values of J₀ at respective temperatures are shown in thefollowing Table 5. The values thus obtained of J₀ were used to make anArrhenius plot according to Formula (8), and a result thereof is shownin FIG. 23.

TABLE 5 temperatures [° C.] J₀[mA/cm²] −35 7.1 × 10⁻³ −20 1.4 × 10⁻² −53.2 × 10⁻² 10 5.8 × 10⁻² 25 9.0 × 10⁻² 40 1.5 × 10⁻¹ 55 2.2 × 10⁻¹ 702.9 × 10⁻¹ 85 3.6 × 10⁻¹

Since the Arrhenius plot shows linearity as shown in FIG. 23, it issuggested that hole injection from ITSO into DNTPD is a Schottkyinjection mechanism. In addition, it is also found that the Schottkyinjection controls a current amount. Note that a Schottky barrier φ_(B)is found from the slope in FIG. 23 to be 0.20 eV.

Example

Subsequently, current-voltage characteristics of the composite materialof the invention are exemplified. First, a glass substrate, over whichITSO was deposited with a thickness of 110 nm, was prepared. Theperiphery of ITSO was covered with an insulating film so that a portionof the ITSO surface with a size of 2 mm square was exposed.

Next, the glass substrate was fixed to a substrate holder in a vacuumevaporation apparatus so that the side provided with ITSO faceddownward. Then, DNTPD and molybdenum oxide (VI) were separately put indifferent resistance-heating evaporation sources, and the compositematerial of the invention formed form DNTPD and molybdenum oxide wasdeposited under vacuum by a co-evaporation method. At this time, DNTPDwas evaporated at a deposition rate of 0.2 nm/s and an adjustment wasperformed so that DNTPD: molybdenum oxide was 2:1 (mass ratio). Athickness thereof was 200 nm to correspond to Comparative Example.Further, aluminum (Al) was deposited thereover with a thickness of 200nm.

As to the laminated structure obtained thus in which ITSO, a mixed filmof DNTPD and molybdenum oxide, and Al are sequentially laminated overthe substrate, measurement results of current-voltage characteristics at−35° C., −20° C., −5° C., 10° C., 25° C., 40° C., 55° C., 70° C., and85° C. are shown in FIG. 24A. Note that the case where a potential ofITSO is higher than that of Al is regarded as forward bias.

Since almost the same amount of current flows both under forward biasand reverse bias, which is different from the above Comparative Example(FIG. 22A), it is found that an equivalent amount of holes is injectedfrom both ITSO and Al. In addition, it is also found that a largeramount of current flows at low voltage as compared to the aboveComparative Example.

Subsequently, the current-voltage characteristics obtained in FIG. 24Awere analyzed to see whether current flowing in this example wascontrolled by a Schottky injection mechanism (in other words, whether itis controlled by injection). First, under forward bias (in other words,at the time of hole injection from ITSO), J₀ was found as in the aboveComparative Example by replacing the horizontal axis of thecurrent-voltage characteristics obtained in FIG. 24A with a square rootof a voltage V, replacing the vertical axis with a logarithm of acurrent density J (see FIG. 24B), and extrapolating a plot (solid linesin the diagram) at respective temperatures. Subsequently, an Arrheniusplot was made according to Formula (8). A result thereof is shown inFIG. 25.

As shown in FIG. 25, it is found that in the case of using the compositematerial of the invention, an Arrhenius plot of J₀/T² is not linear.This suggests that a Schottky injection mechanism is not dominant as tohole injection from ITSO into the composite material of the invention.

Thus, the current-voltage characteristics of the composite material ofthe invention were analyzed to see whether they followed Formula (1)described in Embodiment Mode 1. A method of analysis is the same as thatdescribed in Embodiment 2. The plot under forward bias in FIG. 24A wasfitted by Formula (9) to find A′ at respective temperatures, and it wasdetermined if A′ followed Formula (11).

A result of fitting the plot under forward bias in FIG. 24A by Formula(9) is shown in FIG. 26. Broken lines in the diagram show the fitting byFormula (9) in the case of n=5. As FIG. 26 shows, it is found that thefitting is performed with extreme precision. Values of A′ found by thisfitting at respective temperatures are shown in the following Table 6.

TABLE 6 temperatures [° C.] A′[mA/cm²/V] −35 1.4 × 10  −20 2.8 × 10  −58.1 × 10² 10 5.0 × 10² 25 1.2 × 10² 40 1.6 × 10² 55 2.2 × 10² 70 2.7 ×10² 85 3.3 × 10²

A result of making an Arrhenius plot of A′ is shown in FIG. 27. Sincethe Arrhenius plot shows linearity as shown in FIG. 27, it is found thatan ohmic contact is formed as to hole injection from ITSO into thecomposite material of the invention and a current amount thereof followsFormula (1). Note that activation energy φ_(a) at this time is 0.37 eV.

Since almost the same amount of current as that under forward bias alsoflows under reverse bias, it is found that an ohmic contact is formed asto hole injection from Al into the composite material of the inventionand a current amount thereof follows Formula (1).

Note that a work function of ITSO is 4.89 eV and that of Al isapproximately 4 eV (each of which is measured using a photoelectronspectrometer AC-2 (manufactured by Riken Keiki Co., Ltd.)). According tothe above, it is found that measured current-voltage characteristics ofthe composite material of the invention sandwiched between electrodeseach having a work function of 3.5 eV to 5.5 eV follow Formula (1).

In addition, following Formula (1) allows the composite material of theinvention to have an ohmic with the electrode and in addition, toconduct a large amount of current. Thus, a light emitting element inwhich the composite material of the invention is provided in contactwith the electrode can reduce a drive voltage. In addition,general-purpose metal such as aluminum can be used for an anode.

1. A light emitting element comprising a first layer and a third layerand a second layer containing a light emitting material between a firstelectrode and a second electrode, wherein the first layer is provided incontact with the first electrode, and the first layer is formed from acomposite material comprising organic compounds and an inorganiccompound, wherein the first electrode forms an ohmic contact with thefirst layer, wherein the composite material is to have both bandconduction using carrier generation by adding an electron-acceptingmaterial to the organic compound and hopping conduction between organiccompounds when the organic compound transports carriers injected fromoutside, wherein the third layer is between the first layer and thesecond layer, wherein the organic compounds are an aromatic aminecompound and rubrene, wherein the inorganic compound is a molybdenumoxide, and wherein the first electrode is an indium tin oxide layer towhich silicon is added.
 2. A light emitting element according to claim1, wherein the first electrode is an anode.
 3. A light emitting elementaccording to claim 1, wherein the first electrode contains a materialhaving a work function of 3.5 eV to 5.5 eV.
 4. A light emitting devicecomprising the light emitting element according to claim
 1. 5. A lightemitting device according to claim 1, wherein the organic compound has ahole-transporting property.
 6. A light emitting device according toclaim 1, wherein the third layer includes an electron transportingorganic compound and a material showing an electron donating property tothe organic compound.
 7. A light emitting device according to claim 1,wherein a measured current-voltage characteristic of a thin-film layerformed from the composite material follows Formula (1); $\begin{matrix}{J = {{\left\{ {A\;{\exp\left( \frac{- \phi_{a}}{2{kT}} \right)}} \right\} V} + {BV}^{n}}} & (1)\end{matrix}$ wherein J denotes a current density; V, a voltage; φ_(a),activation energy for carrier generation in the composite material; k,Boltzmann constant; T, a temperature; A and B are constants determinedby a distance between the pair of electrodes d, the amount of elementarycharge q, a mobility μ determined by the kind of the composite material,a dielectric constant ∈, the number of traps per unit volume Nt, thenumber of LUMO levels per volume of the organic compound in thecomposite material N_(LUMO); and n, an integer of 2 to
 10. 8. A lightemitting device according to claim 1, wherein the aromatic aminecompound is selected from the group consisting of4,4′,4″-tris(N,N-diphenylamino)triphenylamine,4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine,1,3,5-tris[N,N-di(m-tolyl)amino]benzene,N,N′-diphenyl-N,N″-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl,4,4′-bis{N-[4-di(m-tolyl)amino]phenyl-N-phenylamino}biphenyl,4,4′-bis[N-(4-biphenylyl)-N-phenylamino]biphenyl,4,4′,4″-tri(N-carbazolyl)triphenylamine.
 9. A light emitting deviceaccording to claim 1, wherein the light emitting material is selectedfrom the group consisting of 9,10-di(2-naphthyl)anthracene,2-tert-butyl-9,10-di(2-naphthyl)anthracene,4,4′-bis(2,2-diphenylvinyl)biphenyl, Coumarin 30, Coumarin 6, Coumarin545, Coumarin 545T, perylene, rubrene, periflanthene,2,5,8,11-tetra(tert-butyl)perylene, 9,10-diphenylanthracene,5,12-diphenyltetracene,4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran,4-(dicyanomethylene)-2-methyl-6-[2-(julolidine-9-yl)ethenyl]-4H-pyran,4-(dicyanomethylene)-2,6-bis[p-(dimethylamino)styryl]-4H-pyran.
 10. Alight emitting element according to claim 1, wherein the secondelectrode contains a material having a work function of 3.5 eV to 5.5eV.
 11. A light emitting device according to claim 1, wherein the thirdlayer is selected from the group consisting oftris(8-quinolinolato)aluminum, tris(4-methyl-8-quinolinolato)aluminum,bis(10-hydroxybenzo[h]-quinolinato)beryllium,bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum,bis[2-(2′-hydroxyphenyl)benzoxazolato]zinc,bis[2-(2′-hydroxyphenyl)benzothiazolato]zinc, bathophenanthroline,bathocuproin, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole,1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene,2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole,3-(4-biphenylyl)-4-(4-ethylphenyl)-5-(4-tert-butylphenyl)-1,2,4-triazole.12. A light emitting element comprising a first layer and a third layerand a second layer containing a light emitting material between a firstelectrode and a second electrode, wherein the first layer is provided incontact with the first electrode, and the first layer is formed from acomposite material comprising organic compounds and an inorganiccompound, wherein the composite material is to have both band conductionusing carrier generation by adding an electron-accepting material to theorganic compound and hopping conduction between organic compounds whenthe organic compound transports carriers injected from outside, whereinthe third layer is between the first layer and the second layer, whereinthe organic compounds are 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyland rubrene, wherein the inorganic compound is a molybdenum oxide, andwherein the first electrode contains a material having a work functionof 3.5 eV to 5.5 eV.
 13. A light emitting element according to claim 12,wherein the second electrode contains a material having a work functionof 3.5 eV to 5.5 eV.
 14. A light emitting element comprising a firstlayer and a third layer and a second layer containing a light emittingmaterial between a first electrode and a second electrode, wherein thefirst layer is provided in contact with the first electrode, and thefirst layer is formed from a composite material comprising organiccompounds and an inorganic compound, wherein the composite material isto have both band conduction using carrier generation by adding anelectron-accepting material to the organic compound and hoppingconduction between organic compounds when the organic compoundtransports carriers injected from outside, wherein the third layer isbetween the first layer and the second layer, wherein the organiccompounds are 4,4′-bis[N-(4-biphenylyl)-N-phenylamino]biphenyl andrubrene, wherein the inorganic compound is a molybdenum oxide, andwherein the first electrode contains a material having a work functionof 3.5 eV to 5.5 eV.
 15. A light emitting element according to claim 14,wherein the second electrode contains a material having a work functionof 3.5 eV to 5.5 eV.